1. Field of the Invention
This invention relates to a slit illuminating device capable of performing uniform illumination of light at a slit section irrespective of the position of a light source.
2. Description of the Prior Art
There have so far been used generally reflecting mirrors having a quadratic surface such as ellipsoidal, parabolic, and hyperbolic mirrors when viewed in cross-section, or a simple linear surfaced mirror as the reflecting mirror for the slit illuminating device. For the linear surfaced reflecting mirror, there can be enumerated such one as shown in FIG. 1 of this application, and one that consists of a plurality of linear surfaced mirrors as shown in U.S. Pat. No. 3,777,135. In U.S. Pat. No. 3,777,135, divergent light from a light source is reflected by each reflecting mirror, and this reflected light is used as the divergent light for overall irradiation of an image original. Each of the linear surfaced mirrors takes such a position that its light path length may continuously decrease or increase relative to one end of the image original to the other end thereof. However, it does not, at least, take a structure of the quadratic surface having light converging property such as an ellipsoid. This constitutes a point of difference between the slit-exposure and the overall exposure. In other words, while the former is light-converging, the latter is light-diverging. This difference becomes evident in the presence or absence of the light converging property, when compared with a reflection factor of a conventional multi-surfaced mirror aimed at easiness in its manufacture.
In the case of the linear surfaced mirror as shown in FIG. 1, there is such a disadvantage that overall loss in the quantity is great, since light beam from a light source is diverged, although relatively stable distribution of illumination can be obtained at the slit section, even when a light source position substantially deviates from that where it should primarily be.
Also, as shown in FIG. 2, when the reflecting mirror has the quadratic surface in its cross-section such as ellipsoidal, parabolic, and hyperbolic surfaces, there can be performed efficient light illumination once the light source is fixed at the first focus position, because the focus can be determined strictly. On the contrary, when the light source deviates from the particular position, there arises such a disadvantage that the illumination and the distribution of illumination at the slit section varies largely. In order to solve such disadvantage, laid-open Japanese Patent Application No. 51-23725 discloses a reflecting mirror, in which the quadratic surface mirror is slit into two portions, and such split reflecting mirrors are arranged with their focus positions being differentiated. Such reflecting mirror, however, is complicated in its manufacture and adjustment of the focus position, since such double focus position requires adjustment in their inter-relationship. Moreover, there still remains such a question as to whether irregularity in the illumination can be reduced or not, even when the light source position deviates in the direction perpendicular to the principal axis.